Molded articles made of a fiber-reinforced plastic having a closed cross-section are widely used in large molded articles such as bodies and wings of aircrafts to small molded articles such as bicycle frames, tennis rackets, fishing rods and golf shafts. Molded articles made of a fiber-reinforced plastic having an open cross-section are also widely used for helmets and the like.
As a core for forming a closed cross-section, used are those in which foamed materials and/or powder-and-granular materials are wrapped with a packaging film and formed into a predetermined shape. Patent Document 1 proposes a method of manufacturing a fiber-reinforced plastic using a core in which a powder-and-granular material is formed into a desired form.
The invention described in Patent Document 1 will be described with reference to FIG. 5 as a conventional art 1 against the present invention.
FIG. 5 shows a schematic diagram to explain the resin impregnation step and the resin curing step in the method of manufacturing a molded article having a hollow portion using a molding die 108; the hollow portion is a variation of a closed cross-section. That is, a preform 107 produced by winding a reinforcing fiber around a core 104 is placed inside a cavity 109 of the molding die 108 along with the core 104. Here, a connection device (not shown) connectable to an air supply passage (not shown) installed through the molding die 108 is provided at a port of a film bag 105.
Subsequently, when the preform 107 and the core 104 are placed inside the cavity 109, the port of the film bag 105 which has been sealed is opened, and the connection device is attached to the air supply passage installed through the molding die 108, thereby the inside of the film bag 105 is connected. Note that the above air supply passage is connected with an air compressor (not shown), and the air discharged from the air compressor is supplied into the film bag 105 through the air supply passage.
Then, after the preform 107 is placed inside the cavity 109 along with the core 104, a resin injection device 110 is blocked from an injection hole 111 by a three-way valve 113 provided on an injection line 112 connecting the resin injection device 110 with the injection hole 111.
Next, the inside of the cavity 109 is decompressed to near vacuum through a discharging hole 115 by driving a vacuum pump 114 in a state where the vacuum pump 114 is connected with the discharging hole 115 by means of a three-way valve 117 provided in an aspiration line 116 connecting the vacuum pump 114 with the discharging hole 115.
Subsequently, while the inside of the cavity 109 remains decompressed, the resin injection device 110 is connected with the injection hole 111 by means of the three-way valve 113, and a resin impregnation step is performed in which a resin injected from the resin injection device 110 is injected into the cavity 109 through an injection inlet 111a. Note that a curing agent has been added to the resin with which the preform 107 is impregnated.
In the resin impregnation step, the resin injection device 110 discharges the resin at a fixed flow rate and at an injection pressure where the injection pressure of the resin is gradually increased to be a high pressure (for example, 5 MPa) at the end. Here, the injection pressure means a pressure of the resin discharged from the resin injection device 110, and corresponds to the detected pressure by the pressure gage 118.
When the resin is injected into the cavity 109 at a high pressure, the impregnating resin spreads over the entire preform 107. At this time, the core 104, which is hardened since the film bag 105 is filled with particles 106, will not undergo deformation and maintain the predetermined shape even when a high pressure is applied externally during resin injection.
Then, after resin injection is performed with the resin injection device 110 for a predetermined amount of time, the resin injection device 110 and the vacuum pump 114 are stopped. Further, the three-way valves 113 and 117 are opened to the atmosphere to end the resin injection.
Next, a resin curing step is performed to cure the resin. In the resin curing step, after the completion of the resin injection, heating of the resin is started by a heating means (not shown) such as a heater provided inside the molding die 108. Further, the inside of the film bag 105 is pressurized by supplying air into the film bag 105 by means of an air supplying means such as an air compressor.
When air is supplied into the film bag 105, the air passes through the particles 106, and uniformly diffuses over the inside of the film bag 105 as indicated by the dotted arrows in FIG. 5. As a result, the film bag 105 is internally pressurized at a pre-set pressure, and curing of the resin and pressurization of the inside of the film bag 105 are performed in parallel. Then, the film bag 105 uniformly presses against the inner surfaces of the preform 107 with its outer surfaces.
Note that when pressurizing the inside of the film bag 105, a pressure which balances with the pressure resulting from the curing and solidification contraction of the resin used is calculated by conducting preliminary experiments, and then the inside of the film bag 105 is pressurized to the pressure obtained from the experiments. Then, as the curing and solidification contraction of the resin are progressing, the pressure produced by the curing and solidification contraction of the resin will balance with the pressure within the film bag 105 to control the curing and solidification contraction of the resin. Therefore, sink marks which may occur on the surface of the FRP molded article (a phenomenon where a surface condition is roughened due to the failed transfer of the shape of a molding die which may occur when an air layer remains between the molding die and the molded article) can be controlled, or sink marks may be dispersed. Then, a pressure continues to be applied to the inner surfaces of the preform 107 with the film bag 105 until the curing of the resin is completed. When the curing of the resin is completed, heating the resin with the heating means and supplying air into the film bag 105 are stopped.
After the resin curing step is completed, a demolding step is performed for the FRP article. In the demolding step, the molding die 108 is opened, and the connection device provided on the film bag 105 is removed from the air supply passage, and the FRP article is taken out from the molding die 108 along with the core 104. In a step of removing the core 104, the port of the film bag 105 is opened to discharge the air present inside the film bag 105, and then the particles 106 contained in the film bag 105 are removed through the port to shrink the film bag 105. Then, the FRP article in a state where the core 104 is removed is manufactured by removing the film bag 105 from the FRP article.